themes switcher widget: How to choose and confirm the theme ?

themes switcher widget: How to choose and confirm the theme ?

I would like to have the possibility to confirm or not the choice of the user when he clicks on the themes switcher widget.
I tried to intercept the theme selection (with onSelect) but the theme is already changed everywhere !
I can have the name of the theme with the cookie but it's too late !!
Do you know a method to do that ?